Note that you definitely want to make sure you're in the /etc/xdg/autostart directory before running those sed commands.

Some of the apps in /etc/xdg/autostart will still not show up in Startup Applications in Unity. Looking in that directory, a comparison of, for example, bluetooth-applet-unity.desktop and bluetooth-applet.desktop shows why; the former has the line “OnlyShowIn=Unity;” and the latter has:

OnlyShowIn=GNOME;XFCE;

You can make an individual app show up in Startup Applications by going to /etc/xdg/autostart and editing the app's .desktop file to change "NoDisplay=true" to "NoDisplay=false." You can stop the app from autostarting by removing its .desktop file, but that might not be a good idea.

If you use Startup Applications to add an autostart app, there'll be a .desktop file placed in ~/.config/autostart.